1) US-made Pontiacs were open driveline all through the '50s; I'm not sure about Canandian cars, except that as far a I know, the body and running gear were the same as US with different trim
2) If the engine has only 15,000 miles on it, and still turns over, you may be able to bring it back to life. Drain and refill the oil, try to get it running, see what oil pressure it holds, listen for any knocking noises then run a compression test. Your biggest problem will be stuck rings.
3) 6V to 12V is fairly easy, however I run 5 old farm tractors with 6V with no trouble as long as the temperature is over 25 degrees
4) Is the transmission standard or automatic? If it's a standard, it's probably OK.
5) You change the gear ratio in the rear, not the transmission
My recommendation - If it's a stick shift, do your best to get it running and see how it moves. You said you have little mechanical experience and this is a great way to learn. Pontiac parts may be a little harder to find, but the electrical system will be GM (Delco), carbureator parts should be available, and the brakes should be common between Pontiac, Olds, and maybe Buick of that era.
